An ultrasound scanner has been used to estimate a velocity spectrum for flowing structure in an object or subject of interest at a given depth and visually present the velocity distribution as a function of time in a spectrogram. The spectrogram has been calculated by measuring a sampled signal at the given depth and then employing a Fourier transform on the received data. This is discussed in Baker, “Pulsed ultrasonic Doppler blood-flow sensing,” IEEE Trans. Son. Ultrason., SU-17:170-185 (1970), Evans et al., “Doppler Ultrasound, Physics, Instrumentation, and Clinical Applications: John Wiley & Sons, New York (1989), and Jensen, “Estimation of Blood Velocities Using Ultrasound: A Signal Processing Approach,” Cambridge University Press, New York (1996).
For the display, the spectra are stacked side-by-side to show the time evolution of the velocity distribution. The relation between the velocity of the flowing structure and the measured frequency (fp) can be represented as shown in EQUATION 1:
                                          f            p                    =                                                                      2                  ⁢                                      v                    z                                                  c                            ⁢                              f                0                                      =                                                            2                  |                                      v                    →                                    |                                      cos                    ⁢                                                                                  ⁢                    Θ                                                  c                            ⁢                              f                0                                                    ,                            EQUATION        ⁢                                  ⁢        1            where f0 is the frequency of the emitted ultrasound beam, c is the speed of sound, vz is the structure velocity in the axial direction, and Θ is the angle between the structure velocity vector and the ultrasound beam. With this approach, only the axial velocity component is measured, and this measurement should be corrected for the angle Θ. However, when Θ=90 degrees, cos Θ=0, no velocity can be found. As such, this approach cannot be used for measuring velocity in vessels that are transverse to the ultrasound beam direction.